Conventional zircon sand (ZrSiO.sub.4) halogenators, such as chlorinators, produce a product stream of major components, namely zirconium tetrachloride, hafnium tetrachloride and silicon tetrachloride and contaminating minor components. Among the contaminating minor components are volatile chloride compounds which contain iron, aluminum, uranium, phosphorus, titanium, and vanadium. Some finely divided solids, carbon and oxides, are also minor impurities in the product stream. The product stream then contacts a first condenser where ZrCl.sub.4 /HfCl.sub.4 is largely condensed and most SiCl.sub.4 goes further downstream to another condenser. Some of the minor impurities, however, condense with ZrCl.sub.4 /HfCl.sub.4. These impurities, such as chlorides of iron, uranium, and phosphorus, must be removed at some cost in subsequent operations, see for instance those disclosed in German Patent 1,082,240.
One principal object of the invention is to decrease the level of impurities in zirconium/hafnium tetrachloride (ZrCl.sub.4 /HfCl.sub.4) condensed from gas produced by carbochlorination of zircon. Because certain of these impurities require removal in further processing of ZrCl.sub.4 /HfCl.sub.4, their removal at this earlier stage may be advantageous. A reducing gas mixture can be effectively used to react with certain of the impurities forming condensible species which can then be filtered from the gas stream prior to condensation of ZrCl.sub.4 /HfCl.sub.4 (1). These impurities include phosphorus, (1) iron, and uranium. Certain other impurities react to form gaseous products which are more volatile than the ZrCl.sub.4 /HfCl.sub.4 and are non-condensible condensible chlorides.
A still further object of this invention is to provide new processes for decreasing impurity level in the chlorination process for zircon which processes are cost effective, safe, and easily reproducible.
Zirconium occurs naturally together with hafnium, typically such that (100) (Hf/(Zr+Hf)) 2.0. The combination of unseparated zirconium and hafnium has a number of uses as chemicals, e.g., for paper, ceramics and metal matrix composites, or as metal in corrosive environments, e.g., Zircadyne 702 for acetic acid plants.
Nuclear grade zirconium metal (Hf/(Hf+Zr) &lt;100 ppm) alloys, e.g., Zircaloy 4, find application for internal construction materials in nuclear reactors, due to the property of zirconium having a low thermal neutron capture cross section. Hafnium metal finds application as a control rod in nuclear reactors owing to its high thermal neutron capture cross section. Hafnium is also used in superalloys where high temperature strength is needed.
Traditionally, where zirconium/hafnium is destined for use in nuclear applications, its processing begins with the carbochlorination of an ore, typically zircon sand. Zirconium ore may also be opened by a fluoride process. Processing of zirconium and hafnium for non-nuclear applications may also begin with carbochlorination of an ore. The carbochlorination process for zircon (Zr(Hf)SiO.sub.4) sand is based on the process of chlorinating a finely divided zircon sand mixed with coke at temperature near 1000.degree. C. The gaseous product stream ZrCl.sub.4 /HfCl.sub.4 (s.p. .about.331.degree. C.) is filtered to be free of particulate matter and then condensed. Major by-product SiCl.sub.4 (b.p. 57.degree. C.) is recovered downstream in a condenser.
There are a number of impurities in the ZrCl.sub.4 /HfCl.sub.4 product condensed from chlorinator off-gas. The presence of excess chlorine and phosgene create problems, aside from representing wasted chlorine, in that they must be neutralized and the resultant hypochlorite reduced. Further, they are extremely corrosive to materials of construction and represent a potential health hazard. It is difficult to continuously run a chlorinator without some means of escapement of chlorine/phosgene to product. Ferric chloride and aluminum chloride tend to co-condense with ZrCl.sub.4 /HfCl.sub.4 and coat equipment with deposits which impede heat transfer. Uranium, phosphorus, silicon, titanium and compounds may contaminate the product. Traces of carbon and oxide particulate matter also contaminate the product. These contaminating materials, in addition to other contaminating impurities, must also be subsequently removed, and considerable cost and effort is required in the process.
Conventionally, many impurities are removed from crude chloride by a liquid-liquid counter current solvent extraction process (LLCCSE).sup.1 for separating Zr and Hf. One alternative to a LLCCSE is to sublime crude chloride one or more times in a stream of H, in N,. This removes Cl, P, U, Fe, Al, Ti, Si, Cr, V, C and oxides to an acceptable level for feed to any process stream--chemical grade, non-nuclear grade metal and nuclear grade metal. In our experience, the sublimation process is expensive in terms of energy consumed, the need for large scale equipment, and substantial expenses to maintain the equipment. FNT .sup.1 James H. McClain and Stephen Shelton, Ch. 4, Zr/Hf Separation, in Reactor Handbook, Vol. 1. Materials, Ed. by C.R. Tipton, Jr., Interscience Publ, Inc., N.Y., pp. 64-73.
Other attempts to purify ZrCl.sub.4 and/or HfCl.sub.4 by sublimation have been made in the past. In one process, ZrCl.sub.4 was soaked in H.sub.2 at 250.degree. C. prior to sublimation..sup.2 Fe, Cr, U, Th, Si, Ti, and most Al was removed, partially due to reduction of hydrogen reducible elements and simple distillation of impurities that boil or sublime below the s.p. of ZrCl.sub.4. A hydrogen soak is not industrially practical. FNT .sup.2 W. J. Kroll and W.W. Stephens, Production of Malleable Zirconium, Industrial and Engineering Chemistry, Vol. 42, 1950, pp. 395-398.
In still another process, zinc, cadmium and manganese have been used.sup.3 to reduce contaminating iron levels. It was noted that the system should be free of chlorine. Hydrogen sublimation is more industrially practicable. FNT .sup.3 British 660,397, Nov. 7, 1951, Method of Producing Pure Zirconium Halides.
Still numerous other processes are reported. ZrCl.sub.4 was contacted with alkali or alkaline earth chlorides 1. where Fe, Al, Si, Ti, Mg, Ca, Cu, Ba, Na, K, ZrOCl.sub.2 and C were claimed to have been removed..sup.4 HfCl.sub.4 was contacted with a NaCl/KCl/HfCl.sub.4 molten salt bath where Al, Fe, Si, Ti, Mn, Pb and B were largely removed..sup.5 ZrCl.sub.4 was contacted with CaCl, at 200.degree.-450.degree. C. where Al and Fe were removed..sup.6 ZrCl.sub.4 was contacted with a fused salt where Al, Fe, Ti, V, Si and Th were largely removed..sup.7 Contact with KCl, NaCl or NaCl/KCl in a plate column removes Fe and Al and supposedly V, U, Ti and Si..sup.8 The mechanism of removal is the formation of compounds, e.g., NaCl +FeCl.sub.3, NaFeCl.sub.4 which are stable and relatively non-volatile. They require large pieces of equipment and are subject to high maintenance costs. They do not address eliminating chlorine in the off-gas. FNT .sup.4 British 771,144, Mar. 27, 1957, Improvements in or Relating tothe Purification of Zirconium Tetrachloride. FNT .sup.5 D. S. Fairgrieve and J. W. Fortner, Production and Purification of High Purity Hafnium Metal, J. Metals, 12, Jan. 1960, pp. 25-26. FNT .sup.6 Harry Greenberg and Hyman R. Lubowitz, purification Method for Metal Halides, U.S. Pat. No. 3,053,620, Sep. 11, 1962. FNT .sup.7 D. R. Spink, Fused Salt Scrubbing of Zirconium Tetrachloride, Transactions, AIME, Vol. 224, 1962, pp. 965-970. FNT .sup.8 Ernest D. Lee and David F. McLaughlin, Molte Salt Scrubbing of Zirconium or Hafnium tetrachloride, U.S. Pat. No. 4,913,778, Apr. 3, 1990.
Sublimation in the presence of carbonaceous material has been attempted. Fe, Al, V, and Cr impurity levels were reduced when ZrCl.sub.4 was sublimed from an intimate mix of ZrCl.sub.4 and e.g., motor oil..sup.9 Poor ZrCl.sub.4 recovery and a mixing operation are problems with that approach. Sublimation in the presence of a volatile hydrocarbon, such as pure or mixtures of ethane, ethylene, propane, propylene or butane lowered Fe, Al, Ti, and Si levels..sup.10 Addition of steam and a catalytic surface area enhanced this approach..sup.11,12 Chlorinated hydrocarbons produced in this process are problematic due to environmental concerns and potentially costly cleanup efforts. FNT .sup.9 Walter Frey, Process fro the Production of Purified Anhydrous Zirconium Tetrahalide, U.S. Pat. No. 2,682,445, Jun. 28, 1954. FNT .sup.10 Herman Renner, Method for the Production of Pure Zirconium Tetrachloride by Sublimation, German 1,068,683, Nov. 12,1959. FNT .sup.11 Process for the Production of Pure Zirconium Tetrachloride and Hafnium/Zirconium Tetrachloride by Sublimation, British 910,289, Nov. 14, 1962. FNT .sup.12 Hans Herzog and Herman Renner, Method for the Production of Pure Zirconium Tetrachloride by Sublimation, German 1,082,240, May 24, 1960.
While hydrogen sulfide or metallic sulfides in contact with ZrCl.sub.4 during sublimation reduce Fe and V impurity levels.sup.13, the other impurity levels are not addressed. The formation of other sulfides necessarily means that they will have to be rendered innocuous and subsequently disposed of; but this would also require costly processing. FNT .sup.13 James Thompson Richmond and Howard Artner Stanley Bristow, Process for the Purification of Titanium Tetrachloride or Zirconium Tetrachoride, Brit. 866,771, May 3, 1961.
The objects of this invention are dramatically accomplished by the processes disclosed herein.